Overwhelming infection can lead to massive end-organ damage and death, which is a clinical condition known as septic shock. Current concepts of septic shock pathogenesis emphasize the role of innate immunity and excessive inflammatory cytokine production. Under normal circumstances, these responses are carefully regulated so that the infectious agent is eliminated without causing life-threatening tissue damage. Inhibition of inflammatory cytokines has been studied as a way to treat septic shock, however therapies directed at mediators such as TNF alpha and IL-I have not shown clinical benefit, and in fact have worsened outcome in some studies. We have discovered that macrophage migration inhibitory factor (MIF) is an important mediator of innate immunity and upstream regulator of inflammatory cytokine production. Anti-MIF protects from shock caused by live bacteria in the same models of infection that have demonstrated either a null or a detrimental effect of anti-TNF alpha therapy. Interference with MIF action thus offers potential therapeutic advantages that have not been observed with previous anti-cytokine interventions. The objective of this application is to understand the mechanism by which MIF contributes to the overwhelming inflammatory response that produces shock. Our central hypothesis is that septic shock is the result of genetic predisposition to overproduction of MIF, coupled with M1F activation of cells by binding to the surface protein, CD74. We have formulated this hypothesis on the basis of our studies showing, first, that anti-MIF prevents shock in relevant models of bacterial sepsis, second, that MIF is encoded by a functionally polymorphic gene, and third, that MIF initiates signal transduction by binding to CD74. The rationale for this proposed research is that once it is known how MIF expression leads to septic shock, then new and selective approaches for therapeutic intervention may be devised. We will test our hypothesis by pursuing the following three specific aims: 1) Define the Frequency of the Low and High Expression M/f Alleles in Patients with Pneumonia, and Determine if the High Expression Alleles are Associated with Septic Shock. Our working hypothesis is that high expression M/f alleles wilt be over-represented in patients with pneumonia who develop shock. 2) Define the Mechanism of MIF Signal Transduction. Our working hypothesis is that CD74 transduces MIF signals by recruiting an additional protein(s) into the signaling complex. 3) Determine the Biological Significance of the MIF-CD74 Interaction. Our working hypothesis is that anti-CD74 mAb and soluble CD74 protein (sCD74) will protect mice from lethal septic shock. The proposed research is innovative because it capitalizes on two recent and potentially unifying findings, a polymorphism in the M/fgene that affects its level of expression, and a cell surface protein (CD74) that mediates MIF signal transduction. These results will be significant because they will provide a better understanding of the molecular pathways responsible for the development of sepsis in susceptible individuals.